The present invention relates to Ziegler-Natta catalyst compositions for use in the polymerization of ethylene and mixtures of ethylene with one or more C4-8 α-olefins having improved high temperature polymerization properties. More particularly, the present invention relates to such catalyst compositions that are self-limiting or auto-extinguishing, thereby avoiding polymer agglomeration, operability problems, and/or reactor sheeting, chunking or fouling due to localized overheating or even “run-away” polymerizations.
Ziegler-Natta catalyst compositions are well known in the art. Typically, these compositions include a Group 3-10 transition metal containing procatalyst compound, especially a complex of titanium-, halide-, and, optionally magnesium-C1-6 alkoxide- and/or C6-10 aryloxide-moieties; a co-catalyst, usually an organoaluminum compound, especially a trialkylaluminum compound; and a support, preferably finely divided magnesium dichloride. Non-limiting examples of suitable Group 4 metal complexes that are useful as procatalysts include TiCl4, TiCl3, Ti(OC2H5)3Cl, Ti(OC2H5)2Cl2, Ti(OC2H5)3Cl, Ti(OC3H7)Cl3, Ti(OC3H7)2Cl2, Ti(OC4H9)Cl3, Ti(OC4H9)2Cl2, TiCl3.1/3AlCl3, Ti(OC12H25)Cl3, MgTi(OC2H5)5Cl, MgTi(OC2H5)4Cl2, MgTi(OC2H5)3Cl3, MgTi(OC2H5)2Cl4, MgTi(OC2H5)Cl5, and mixtures thereof. Additional suitable components of the Ziegler-Natta catalyst composition may include an internal electron donor, especially C1-6 alkyl esters of aromatic carboxylic or dicarboxylic acids; dispersants; surfactants; diluents; inert supports such as silica or alumina; binding agents; and antistatic compounds. Examples of Ziegler-Natta catalyst compositions are shown in U.S. Pat. Nos. 4,107,413; 4,115,319; 4,220,554; 4,294,721; 4,302,565; 4,302,566; 4,330,649; 4,439,540; 4,442,276; 4,460,701; 4,472,521; 4,540,679; 4,547,476; 4,548,915; 4,562,173; 4,728,705; 4,816,433; 4,829,037; 4,927,797; 4,990,479; 5,028,671; 5,034,361; 5,066,737; 5,066,738; 5,077,357; 5,082,907; 5,106,806; 5,146,028; 5,151,399; 5,153,158; 5,229,342; 5,247,031; and 5,247,032.
In a typical continuous gas phase polymerization process, fouling or sheeting can lead to the ineffective operation of various reactor components. For example, accumulation of solid polymer on the surfaces of the reactor, the distributor plate, monitoring sensors, and the recycle system can lead to difficulty in operation and an early reactor shutdown. This problem is often encountered during polymerization of ethylene and ethylene/C4-8 α-olefin mixtures since the polymerization reaction is typically conducted at temperatures that are relatively close to the softening temperature or melting point of the resulting polymer.
Reasons for the occurrence of sheeting or fouling and solutions to the various process operability problems caused thereby have been addressed by many in the art. For example, U.S. Pat. Nos. 4,792,592, 4,803,251, 4,855,370 and 5,391,657 all discuss techniques for reducing static generation, and ultimately fouling in a gas phase polymerization process by use of water, alcohols, ketones, and/or inorganic chemical additives. WO 97/14721 discusses the suppression of fines for the same purpose by adding an inert hydrocarbon to the reactor. U.S. Pat. No. 5,627,243 discusses a new type of distributor plate for use in fluidized bed gas phase reactors. WO 96/08520 discusses avoiding the introduction of a scavenger into the reactor. U.S. Pat. No. 5,461,123 discusses using sound waves to reduce sheeting. U.S. Pat. No. 5,066,736 and EP-A1 0 549 252 discuss the introduction of an activity retarder to the reactor to reduce agglomerates. U.S. Pat. No. 5,610,244 relates to feeding make-up monomer directly into the reactor above the bed to avoid fouling and improve polymer quality. U.S. Pat. No. 5,126,414 discusses oligomer removal for reducing distributor plate fouling. U.S. Pat. No. 4,012,574 discusses adding a surface-active compound, such as a perfluorocarbon group, to the reaction mixture. U.S. Pat. Nos. 5,026,795, 5,410,002, 5,034,480, 3,082,198 and EP-A453,116 disclose the addition of various antistatic agents to the polymerization zone in the reactor to reduce fouling, among other reasons.
There are various other known methods for improving reactor operability including coating the polymerization equipment, for example, treating the walls of a reactor using chromium compounds as described in U.S. Pat. Nos. 4,532,311 and 4,876,320, and feeding the catalyst into particle lean regions of the reactor, as discussed in WO 97/46599. Other known methods of reducing fouling include injecting antifoulants or antistatic agents into the reactor; controlling the polymerization rate in the reaction zone; reconfiguring the reactor design; modifying the catalyst system by combining the catalyst components in a particular order; manipulating the ratio of the various catalyst components; prepolymerizing a portion of the monomer; and varying the contact time and/or temperature when combining the components of a catalyst composition. Examples of the foregoing techniques include: WO 96/11961 (use of an antistatic agent); U.S. Pat. No. 5,283,218 (prepolymerization); and U.S. Pat. No. 4,942,147 and 5,362,823 (addition of autoacceleration inhibitors). With respect to the latter two patents, suitable autoacceleration inhibitors were stated to be Diels-Alder adducts that decomposed at elevated temperatures thereby generating a poison for the catalyst composition, exemplified by vanadium based catalysts. In U.S. Pat. No. 6,180,735, the use of solid carbonyl compounds, including aromatic carboxylic acid esters (col. 18, line 57) as one component of a catalyst composition for olefin polymerization was disclosed.
While all these possible solutions might reduce the level of fouling or sheeting of a gas phase polymerization somewhat, some are expensive to employ, some require the addition of undesirable foreign materials into the reactor, some require constant monitoring by an operator and additions in amounts and at times that must be determined empirically, and some may not reduce fouling or sheeting adequately or quickly enough for commercial purposes.
Thus, it would be advantageous to have a gas-phase polymerization process capable of operating continuously with enhanced reactor operability. In particular, the industry still desires a continuously operating, gas phase, Ziegler-Natta catalyzed, olefin polymerization process having reduced fouling or sheeting tendency, and increased duration of operation.
In particular, there remains a need in the art to provide an olefin polymerization process using a Ziegler-Natta catalyst composition for the polymerization of ethylene or mixtures of ethylene with one or more C4-8 α-olefins, which process has improved, self-limiting or auto-extinguishing properties, resulting in effectively reduced catalytic activity and heat generation, especially at elevated reaction temperatures. Desirably, the reduction in polymerization activity is inherent in the catalyst composition and does not require monitoring and outside intervention by an operator. In addition, there remains a need in the art to provide an additive or component (polymerization control agent or PCA) for use in combination with an olefin polymerization catalyst composition that is able to result in the foregoing improved, self-limiting, polymerization process.